Many satellite communication systems may use devices known as single reflector antennas as the means of sending electromagnetic signals. Such antennas may include a reflector surface, either paraboloid or otherwise shaped, and a feed placed at or near the reflector focus. The antenna may operate in a receiving mode, transmitting mode, or both simultaneously. The electromagnetic energy received or transmitted by the antenna may be collimated into a narrow beam and directed from the satellite towards a specified location on the earth surface. This location may be fixed for the duration of the mission, except for minor adjustments, in which case the antenna structure and the mounting method is static and relatively simple. However, very often the antenna direction of radiation may vary, either because the requirements of the mission have changed, or because the intended target travels as a function of time. The antenna needs to be steered to direct the beam towards a specified location. Such steerable antennas have to incorporate special features in their mechanical and electrical design in order to perform their function.
Current implementation options for steerable beam antennas are principally governed by tradeoffs of performance/functionality vs. cost/mass/volume. The antenna designer may be faced first with two main choices. One is a fully steerable system, where the reflector and the feed form a single mechanical assembly, are placed together on a gimbal steering mechanism, and controlled as a unit. This type of system offers the best performance, virtually invariable with the scan angle. However, it may have two main drawbacks. First, it may require an RF rotary joint or a flexible waveguide connection at the interface between the steerable antenna and the RF transponder circuitry. Solutions to this RF interface issue have been addressed by installing the RF transponder circuitry onto the antenna eliminating the need for a flexible interface, but this may limit the utility and may result in significant increases in deployed/gimbaled mass. Second, such a solution may be unacceptably costly to implement, and may require large volume, mass, and sturdy gimbal mechanisms. Third, stowage of multiple full steered antennas can be problematic, driving spacecraft launch vehicle faring size and cost. Achieving sufficiently high rates of motion, meeting acceleration/deceleration limits, and ensuring cycle lifetimes may all be very difficult. For these reasons, with the exception of small steerable antennas, fully steerable systems are rarely practical.
The second choice is a system with an independently steerable reflector and a fixed feed. In this type of steerable antenna, only the reflector is placed on a gimbal steering mechanism. The feed is mounted on the satellite body and may not require a rotary joint for its connection to the transponder. Since the reflector mass is relatively small, it is possible to use economic light-weight gimbals, achieve high rates of motion, and long cycle lifetimes. However, a steerable antenna with a rotating reflector and a fixed feed may suffer from a loss of performance (e.g., decrease in peak gain and changes in the beam shape) as the steering angle increases. This loss of performance is usually referred to as the scan loss. When the reflector rotates in order to steer the beam towards the desired direction, the focal point of the reflector may move away from the fixed feed, and the ray relationship between the feed and the reflector may gradually become less optimal. For large diameter antennas that need to steer over a wide range of scan angles, the scan loss may be high (2-5 dB as an example) and therefore prohibitive. Nevertheless, the systems with an independently steered reflector and a fixed feed are often the only practical option.
For the steerable antenna systems using a steerable reflector and a fixed feed, there are in turn two main design options, again trading off performance vs. cost/mass/volume. The first design option is a reflector rotated about center, where the gimbal mechanism is placed behind the reflector surface, with the center of rotation near or in the vicinity of the aperture center. Since the reflector center is then approximately stationary, and the movement of the reflector rim relative to the feed is minimized, the scan loss may be minimized. However, placing the gimbal at the aperture center, which usually means away from the spacecraft body, is often difficult to implement, requires additional mass and volume, and may be impossible to accommodate for multiple reflectors systems stowed in an overlapped configuration.
The second design option is a reflector rotated about vertex, where the gimbal mechanism is placed in the vicinity of the reflector vertex. This is the most convenient location from the viewpoint of mechanical implementation, with the gimbal located close to the spacecraft body, allowing a compact, low mass, low cost solution. This approach allows for more compact stowage, and enables stowage of multiple nested reflectors along a single side of the spacecraft. However, because the reflector displacement relative to the feed is larger than for the reflector rotated about the center, the scan loss for this method is unfortunately much higher. In spite of the advantages of its mechanical implementation, the scan performance of a reflector steered about its vertex, for the same range of scan angles, is usually inferior.